TDD FDD Communication Interface

ABSTRACT

TDD access equipment modified by introducing a frequency change at the normal TDD guard point, with respective downlink or uplink periods for individual subscriber stations offset to form overlapping frames. Cyclo-stationary processing, block equalization, and burst timing coordination allow the boundary between downlink and uplink portions of both frames to be set dynamically, improving spectral efficiency. Fast frequency switching within an allotted physical slot enables synchronization of time-sharing the dedicated frequencies to be maintained among subscriber stations. Duplex spacing between downlink and uplink frequencies for individual subscriber stations, combined with in-depth filtering of received signals, prevents spurious out-of-band transmission signal strength from reaching an interference level.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of prior U.S. patent application Ser.No. 11/982,404 filed on Oct. 31, 2007, which claims priority to: U.S.patent application Ser. No. 09/839,499 filed on Apr. 20, 2001, now U.S.Pat. No. 7,346,347. U.S. patent application Ser. No. 09/839,499 claimspriority to provisional U.S. Patent Application Ser. No. 60/262,708filed on Jan. 19, 2001 and U.S. patent application Ser. No. 09/713,684filed on Nov. 15, 2000.

This application claims priority to: provisional U.S. Patent ApplicationSer. No. 60/262,712 filed on Jan. 19, 2001 and entitled “WIRELESSCOMMUNICATION SYSTEM USING BLOCK FILTERING AND FAST EQUALIZATIONDEMODULATION AND METHOD OF OPERATION”; provisional U.S. PatentApplication Ser. No. 60/262,825 filed on Jan. 19, 2001 and entitled“APPARATUS AND ASSOCIATED METHOD FOR OPERATING UPON DATA SIGNALSRECEIVED AT A RECEIVING STATION OF A FIXED WIRELESS ACCESS COMMUNICATIONSYSTEM”; provisional U.S. Patent Application Ser. No. 60/262,698 filedon Jan. 19, 2001 and entitled “APPARATUS AND METHOD FOR OPERATING ASUBSCRIBER INTERFACE IN A FIXED WIRELESS SYSTEM”; provisional U.S.Patent Application Ser. No. 60/262,827 filed on Jan. 19, 2001 entitled“APPARATUS AND METHOD FOR CREATING SIGNAL AND PROFILES AT A RECEIVINGSTATION”; provisional U.S. Patent Application Ser. No. 60/262,826 filedon Jan. 19, 2001 and entitled “SYSTEM AND METHOD FOR INTERFACE BETWEEN ASUBSCRIBER MODEM AND SUBSCRIBER PREMISES INTERFACE”; provisional U.S.Patent Application Ser. No. 60/262,951 filed on Jan. 19, 2001 entitled“BACKPLANE ARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESSSYSTEMS”; provisional U.S. Patent Application Ser. No. 60/262,824 filedon Jan. 19, 2001 entitled “SYSTEM AND METHOD FOR ON-LINE INSERTION OFLINE REPLACEABLE UNITS IN WIRELESS AND WIRELINE ACCESS SYSTEMS”;provisional U.S. Patent Application Ser. No. 60/263,101 filed on Jan.19, 2001 entitled “SYSTEM FOR COORDINATION OF TDD TRANSMISSION BURSTSWITHIN AND BETWEEN CELLS IN A WIRELESS ACCESS SYSTEM AND METHOD OFOPERATION”; provisional U.S. Patent Application Ser. No. 60/263,097filed on Jan. 19, 2001 and entitled “REDUNDANT TELECOMMUNICATION SYSTEMUSING MEMORY EQUALIZATION APPARATUS AND METHOD OF OPERATION”;provisional U.S. Patent Application Ser. No. 60/273,579 filed Mar. 5,2001 and entitled “WIRELESS ACCESS SYSTEM FOR ALLOCATING ANDSYNCHRONIZING UPLINK AND DOWNLINK OF TDD FRAMES AND METHOD OFOPERATION”; provisional U.S. Patent Application Ser. No. 60/262,955filed Jan. 19, 2001 and entitled “TDD FDD AIR INTERFACE”; provisionalU.S. Patent Application Ser. No. 60/262,708 filed on Jan. 19, 2001 andentitled “APPARATUS, AND AN ASSOCIATED METHOD FOR PROVIDING WLAN SERVICEIN A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”; provisional U.S.Application Ser. No. 60/273,689 filed on Mar. 5, 2001 and entitled“WIRELESS ACCESS SYSTEM USING MULTIPLE MODULATION FORMATS IN TDD FRAMESAND METHOD OF OPERATION”; provisional U.S. Patent Application Ser. No.60/273,757 filed Mar. 5, 2001 and entitled “WIRELESS ACCESS SYSTEM ANDASSOCIATED METHOD USING MULTIPLE MODULATION FORMATS IN TDD FRAMESACCORDING TO SUBSCRIBER SERVICE TYPE”; provisional U.S. PatentApplication Ser. No. 60/270,378 filed Feb. 21, 2001 and entitled“APPARATUS FOR ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESSCOMMUNICATION SYSTEM”; provisional U.S. Patent Application Ser. No.60/270,385 filed Feb. 21, 2001 and entitled, “APPARATUS FOR REALLOCATINGCOMMUNICATION RESOURCES TO ESTABLISH A PRIORITY CALL IN A FIXED WIRELESSACCESS COMMUNICATION SYSTEM”; and provisional U.S. Patent ApplicationSer. No. 60/270,430 filed Feb. 21, 2001 and entitled “METHOD FORESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS COMMUNICATIONSYSTEM. Each of these applications and/or patents are herebyincorporated by reference herein in their entireties.

In addition, the subject matter disclosed in the present application isrelated to that disclosed in the following U.S. patent applications:

-   1) Ser. No. 10/042,705, filed on Nov. 15, 2000, entitled “SUBSCRIBER    INTEGRATED ACCESS DEVICE FOR USE IN WIRELESS AND WIRELINE ACCESS    SYSTEMS”, now abandoned;-   2) Ser. No. 09/838,810 filed Apr. 20, 2001, and entitled “WIRELESS    COMMUNICATION SYSTEM USING BLOCK FILTERING AND FAST    EQUALIZATION-DEMODULATION AND METHOD OF OPERATION”, now U.S. Pat.    No. 7,075,967;-   3) Ser. No. 09/839,726 filed Apr. 20, 2001, and entitled “APPARATUS    AND ASSOCIATED METHOD FOR OPERATING UPON DATA SIGNALS RECEIVED AT A    RECEIVING STATION OF A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”,    now U.S. Pat. No. 7,099,383;-   4) Ser. No. 09/839,729 filed Apr. 20, 2001, and entitled “APPARATUS    AND METHOD FOR OPERATING A SUBSCRIBER INTERFACE IN A FIXED WIRELESS    SYSTEM”, abandoned;-   5) Ser. No. 09/839,719 filed Apr. 20, 2001, and entitled “APPARATUS    AND METHOD FOR CREATING SIGNAL AND PROFILES AT A RECEIVING STATION”,    now U.S. Pat. No. 6,947,477;-   6) Ser. No. 09/838,910 filed Apr. 20, 2001, and entitled “SYSTEM AND    METHOD FOR INTERFACE BETWEEN A SUBSCRIBER MODEM AND SUBSCRIBER    PREMISES INTERFACES”, now U.S. Pat. No. 6,564,051;-   7) Ser. No. 09/839,509 filed Apr. 20, 2001 and entitled “BACKPLANE    ARCHITECTURE FOR USE IN WIRELESS AND WIRELINE ACCESS SYSTEMS”,    abandoned;-   8) Ser. No. 09/839,514 filed Apr. 20, 2001, and entitled “SYSTEM AND    METHOD FOR ON-LINE INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS    AND WIRELINE ACCESS SYSTEMS”, now U.S. Pat. No. 7,069,047;-   9) Ser. No. 09/839,512 filed Apr. 20, 2001, and entitled “SYSTEM FOR    COORDINATION OF TDD TRANSMISSION BURSTS WITHIN AND BETWEEN CELLS IN    A WIRELESS ACCESS SYSTEM AND METHOD OF OPERATION”, now U.S. Pat. No.    6,804,527;-   10) Ser. No. 09/839,259 filed Apr. 20, 2001, and entitled “REDUNDANT    TELECOMMUNICATION SYSTEM USING MEMORY EQUALIZATION APPARATUS AND    METHOD OF OPERATION”, now U.S. Pat. No. 7,065,098;-   11) Ser. No. 09/839,457 filed Apr. 20, 2001, and entitled    “WIRELESS′ACCESS SYSTEM FOR ALLOCATING AND SYNCHRONIZING UPLINK AND    DOWNLINK OF TDD FRAMES AND METHOD OF OPERATION”, now U.S. Pat. No.    7,002,929;-   12) Ser. No. 09/839,075 filed Apr. 20, 2001, and entitled “TDD FDD    AIR INTERFACE”, now U.S. Pat. No. 6,859,655;-   13) Ser. No. 09/839,458 filed. Apr. 20, 2001, and entitled “WIRELESS    ACCESS SYSTEM USING MULTIPLE MODULATION FORMATS IN TDD FRAMES AND    METHOD OF OPERATION”, now U.S. Pat. No. 7,173,916;-   14) Ser. No. 09/839,456 filed Apr. 20, 2001, and entitled “WIRELESS    ACCESS SYSTEM AND ASSOCIATED METHOD USING MULTIPLE MODULATION    FORMATS IN TDD FRAMES ACCORDING TO SUBSCRIBER SERVICE TYPE”, now    U.S. Pat. No. 6,891,810;-   15) Ser. No. 09/838,924 filed Apr. 20, 2001, and entitled “APPARATUS    FOR ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS    COMMUNICATION SYSTEM”, now U.S. Pat. No. 7,274,946;-   16) Ser. No. 09/839,727 filed Apr. 20, 2001, and entitled “APPARATUS    FOR REALLOCATING COMMUNICATION RESOURCES TO ESTABLISH A PRIORITY    CALL IN A FIXED WIRELESS ACCESS COMMUNICATION SYSTEM”, now U.S. Pat.    No. 7,031,738;-   17) Ser. No. 09/839,734 filed Apr. 20, 2001, and entitled “METHOD    FOR ESTABLISHING A PRIORITY CALL IN A FIXED WIRELESS ACCESS    COMMUNICATION SYSTEM”, now U.S. Pat. No. 7,035,241;-   18) Ser. No. 09/839,513 filed Apr. 20, 2001, and entitled “SYSTEM    AND METHOD FOR PROVIDING AN IMPROVED COMMON CONTROL BUS FOR USE IN    ON-LINE INSERTION OF LINE REPLACEABLE UNITS IN WIRELESS AND WIRELINE    ACCESS SYSTEMS”, now U.S. Pat. No. 6,925,516; and-   19) Ser. No. 09/948,059, filed Sep. 5, 2001, and entitled “WIRELESS    ACCESS SYSTEM USING SELECTIVELY ADAPTABLE BEAM FORMING IN TDD FRAMES    AND METHOD OF OPERATION”, now U.S. Pat. No. 7,230,931.

The above applications are commonly assigned to the assignee of thepresent application. The disclosures of these related patentapplications may share common subject matter and figures and are herebyincorporated by reference herein in their entireties.

The present disclosure is directed, in general, to communication networkaccess systems and, more specifically, to access equipment for use inboth wireless and wireline telecommunications systems.

BACKGROUND

Telecommunications access systems provide for voice, data, andmultimedia transport and control between the central office (CO) of thetelecommunications service provider and the subscriber (customer)premises. Prior to the mid-1970s, the subscriber was provided phonelines (e.g., voice frequency (VF) pairs) directly from the Class 5switching equipment located in the central office of the telephonecompany. In the late 1970s, digital loop carrier (DLC) equipment wasadded to the telecommunications access architecture. The DLC equipmentprovided an analog phone interface, voice CODEC, digital datamultiplexing, transmission interface, and control and alarm remotelyfrom the central office to cabinets located within business andresidential locations for approximately 100 to 2000 phone lineinterfaces. This distributed access architecture greatly reduced linelengths to the subscriber and resulted in significant savings in bothwire installation and maintenance. The reduced line lengths alsoimproved communication performance on the line provided to thesubscriber.

By the late 1980s, the limitations of data modem connections over voicefrequency (VF) pairs were becoming obvious to both subscribers andtelecommunications service providers. ISDN (Integrated Services DigitalNetwork) was introduced to provide universal 128 kbps service in theaccess network. The subscriber interface is based on 64 kbpsdigitization of the VF pair for digital multiplexing into high speeddigital transmission streams (e.g. t TI/T3 lines in North America tE1/E3 lines in Europe). ISDN was a logical extension of the digitalnetwork that had evolved throughout the 1980s. The rollout of ISDN inEurope was highly successful. However, the rollout in the United Stateswas not successful t due in part to artificially high tariff costs whichgreatly inhibited the acceptance of ISDN.

More recently, the explosion of the Internet and deregulation of thetelecommunications industry have brought about a broadband revolutioncharacterized by greatly increased demands for both voice and dataservices and greatly reduced costs due to technological innovation andintense competition in the telecommunications marketplace. To meet thesedemands, high speed DSL (digital subscriber line) modems and cablemodems have been developed and introduced. The DLC architecture wasextended to provide remote distributed deployment at the neighborhoodcabinet level using DSL access multiplexer (DSLAM) equipment. Theincreased data rates provided to the subscriber resulted in upgradeDLC/DSLAM transmission interfaces from T1/E1 interfaces (1.5/2.0 Mbps)to high speed DS3 and OC3 interfaces. In a similar fashion, the entiretelecommunications network backbone has undergone and is undergoingcontinuous upgrade to wideband optical transmission and switchingequipment.

Similarly, wireless access systems have been developed and deployed toprovide broadband access to both commercial and residential subscriberpremises. Initially, the market for wireless access systems was drivenby rural radiotelephony deployed solely to meet the universal servicerequirements imposed by government (i.e., the local telephone company isrequired to serve all subscribers regardless of the cost to installservice). The cost of providing a wired connection to a small percentageof rural subscribers was high enough to justify the development andexpense of small-capacity wireless local loop (WLL) systems.

Deregulation of the local telephone market in the United States (e.g.,Telecommunications Act of 1996) and in other countries shifted the focusof fixed wireless access (FWA) systems deployment from rural access tocompetitive local access in more urbanized areas. In addition, the ageand inaccessibility of much of the older wired telephone infrastructuremakes FWA systems a cost-effective alternative to installing new, wiredinfrastructure. Also, it is more economically feasible to install FWAsystems in developing countries where the market penetration is limited(i.e., the number and density of users who can afford to pay forservices is limited to small percentage of the population) and therollout of wired infrastructure cannot be performed profitably. Ineither case, broad acceptance of FWA systems requires that the voice anddata quality of FWA systems must meet or exceed the performance of wiredinfrastructure.

Wireless access systems must address a number of unique operational andtechnical issues including:

1) Relatively high hit error rates (BER) compared to wire line oroptical systems; and

2) Transparent operation with network protocols and protocol timeconstraints for the following protocols:

-   -   a) ATM;    -   b) Class 5 switch interfaces (domestic GR-303 and international        VS.2);    -   c) TCP/IP with quality-of-service QoS for voice over IP (VoIP)        (i.e., RTP) and other H.323 media services;    -   d) Distribution of synchronization of network time out to the        subscribers;

3) Increased use of voice, video and/or media compression andconcentration of active traffic over the air interface to conservebandwidth;

4) Switching and routing within the access system to distribute signalsfrom the central office to multiple remote cell sites containingmultiple cell sectors and one or more frequencies of operation persector; and

5) Remote support and debugging of the subscriber equipment, includingremote software upgrade and provisioning.

Unlike physical optical or wire systems that operate at bit error rates(BER) of 10-11, wireless access systems have time varying channels thattypically provide bit error rates of 10-3 to 10-6. The wireless physical(PHY) layer interface and the media access control (MAC) layer interfacemust provide modulation, error correction and ARQ protocol that candetect and, where required, correct or retransmit corrupted data so thatthe interfaces at the network and at the subscriber site operate at wireline bit error rates.

Wireless access systems, as well as other systems which employ a sharedcommunications media, must also provide a mechanism for allocatingavailable communications bandwidth among multiple transmitting andreceiving groups. Many wireless systems employ either a time divisionduplex (TDD) time division multiple access (TDMA) or a frequencydiversity duplex (FDD) frequency division multiple access (FDMA)allocation scheme illustrated by the timing diagram of FIGS. 3A and 3B.TDD 300 shares a single radio frequency (RF) channel F1 between the baseand subscriber, allocating time slices between the downlink 301(transmission from the base to the subscriber) and the uplink 302(transmission from the subscriber to the base). FDD 310 employs twofrequencies F1 and F2, each dedicated to either the downlink 311 or theuplink 312 and separated by a duplex spacing 313.

For wireless access systems which provide Internet access in addition toor in lieu of voice communications, data and other Web basedapplications dominate the traffic load and connections within thesystem. Data access is inherently asymmetric, exhibiting typicaldownlink-to-uplink ratios of between 4:1 and 14:1.

TDD systems, in which the guard point (the time at which changeover fromthe downlink 301 to the uplink 302 occurs) within a frame may be shiftedto alter the bandwidth allocation between the downlink 301 and theuplink 302, have inherent advantages for data asymmetry and efficientuse of spectrum in providing broadband wireless access. TDD systemsexhibit 40% to 90% greater spectral efficiency for asymmetric datacommunications than FDD systems, and also support shifting of power andmodulation complexity from the subscriber unit to the base to lowersubscriber equipment costs.

Within the spectrum allocated to multi-channel multipoint distributionsystems (MMDS), however, some spectrum is regulated for only FDDoperation. Since the total spectrum allocated to MMDS is relativelysmall (2.5-2.7 GHz, or about 30 6 MHz channels), some service providersmay desire to utilize the FDD-only spectrum, preferably utilizing theTDD-based equipment employed in other portions of the MMDS spectrum.

There is, therefore, a need in the art for enabling TDD-based equipmentto operate utilizing frequencies reserved for FDD only operation.

In one embodiment the present disclosure provides a method of TDDoperation in a first subscriber unit having a cable modem. The methodincludes receiving in the first subscriber unit a first signal from afirst base station on a downlink frequency during a first time period.The method includes transmitting from the first subscriber unit a secondsignal to the first base station on an uplink frequency during a secondtime period following the first time period. The downlink frequency andthe uplink frequency are separated by a predefined duplex spacing.

In another embodiment, the present disclosure provides a cable modemconfigured to use TDD in a first subscriber station. The modem includesa receiver circuit to receive a first signal on a first frequencydesignated for downlink transmission during a first time period. Themodem also includes a transmitter circuit to transmit a second signal ona second frequency different from the first frequency and designated foruplink transmission during a second time period following the first timeperiod. The first frequency is employed for downlink transmission to asecond subscriber station during the second time period and the secondfrequency is employed for uplink transmission from the second subscriberstation during the first time period.

In still another embodiment, the present disclosure provides a method oftime sharing frequencies reserved for FDD operation for use inconjunction with a cable modem. The method includes receiving in thecable modem a first signal from a first base station on a downlinkfrequency during a first time period. The method also includestransmitting from the cable modem a second signal to the first basestation on an uplink frequency during a second time period following thefirst time period, wherein the downlink frequency and the uplinkfrequency are separated by a predefined duplex spacing.

In a further embodiment, the present disclosure provides a cable modemconfigured to use TDD. The cable modem includes a transmitter circuitconfigured to transmit a first signal on a first frequency to a firstsubscriber station during a first time period and transmit a secondsignal on a second frequency, different from the first frequency, to asecond subscriber station during a second time period following thefirst time period. The cable modem also includes a receiver circuitconfigured to receive a third signal on the second frequency from thesecond subscriber station during the first time period and receive afourth signal on the first frequency from the first subscriber duringthe second time period.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, wherein likenumbers designate like objects, and in which:

FIG. 1 illustrates exemplary wireless access network 100 according toone embodiment of the present disclosure;

FIGS. 2A-2B depict cell and sector layouts for a wireless accesscoverage area according to various embodiments of the presentdisclosure;

FIGS. 3A-3E are comparative high level timing diagrams illustrating thebandwidth allocation among sectors and cells according to the prior artand according to one embodiment of the present disclosure;

FIG. 4 depicts in greater detail a frame structure employed within theexemplary bandwidth allocation scheme according to one embodiment of thepresent disclosure;

FIG. 5 is functional diagram of filtering employed for wirelesscommunication within each cell and sector in accordance with oneembodiment of the present disclosure;

FIG. 6 illustrates a spectral response for filtering employed forwireless communication within each cell and sector in accordance withone embodiment of the present disclosure; and

FIG. 7 is functional diagram of filtering employed for wirelesscommunication within each cell and sector in accordance with anotherembodiment of the present disclosure.

DETAILED DESCRIPTION

FIGS. 1 through 7, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless access network or in timedivision duplex (TDD) wireline applications such as, for example, cablemodems.

FIG. 1 illustrates an exemplary fixed wireless access network 100according to one embodiment of the present disclosure. Fixed wirelessnetwork 100 comprises a plurality of transceiver base stations,including exemplary transceiver base station 110, that transmit forwardchannel (i.e., downstream) broadband signals to a plurality ofsubscriber premises, including exemplary subscriber premises 121, 122and 123, and receive reverse channel (i.e., upstream) broadband signalsfrom the plurality of subscriber premises. Subscriber premises 121-123transmit and receive via fixed, externally-mounted antennas 131-133,respectively. Subscriber premises 121-123 may comprise many differenttypes of residential and commercial buildings, including single familyhomes, multi-tenant offices, small business enterprises (SBE), mediumbusiness enterprises (MBE), and so-called “SOHO” (small office/homeoffice) premises.

The transceiver base stations, including transceiver base station 110,receive the forward channel signals from external network 150 andtransmit the reverse channel signals to external network 150. Externalnetwork 150 may be, for example, the public switched telephone network(PSTN) or one or more data networks, including the Internet orproprietary Internet protocol (IP) wide area networks (WANs) and localarea networks (LANs). Exemplary transceiver base station 110 is coupledto RF modem 140, which, among other things, up-converts baseband datatraffic received from external network 150 to RF signals transmitted inthe forward channel to subscriber premises 121-123. RF modem 140 alsodown-converts RF signals received in the reverse channel from subscriberpremises 121-123 to baseband data traffic that is transmitted toexternal network 150. In an exemplary embodiment of the presentdisclosure in which external network 150 is the public switchedtelephone network (PSTN), RF modem 140 transmits baseband data trafficto, and receives baseband data traffic from, access processor 165, whichis disposed in central office facility 160 of the PSTN.

It should be noted that network 100 was chosen as a fixed wirelessnetwork only for the purposes of simplicity and clarity in explaining asubscriber integrated access device according to the principles of thepresent disclosure. The choice of a fixed wireless network should not beconstrued in any manner that limits the scope of the present disclosurein any way. As will be explained below in greater detail, in alternateembodiments of the present disclosure, a subscriber integrated accessdevice according to the principles of the present disclosure may beimplemented in other types of broadband access systems, includingwireline systems (i.e., digital subscriber line (DSL), cable modem,fiber optic, and the like) in which a wireline connected to thesubscriber integrated access device carries forward and reverse channelsignals.

FIG. 2A depicts a cell and sector layout for a wireless access coveragearea according to one embodiment of the present disclosure. Coveragearea 200 is logically divided into cells 210, 220, 230 and 240 eachlogically divided into a number of sectors 211-216, 221-226, 231-236 and241-246, respectively. Each cell 210, 220, 230 and 240 includes atransceiver base station 110 as depicted in FIG. 1 at a central location217, 227, 237, and 247, respectively, as well as subscriber premises121-123 within the coverage area of the respective cell.

Sectors 211-216, 221-226, 231-236 and 241-246 are logically divided intotwo categories: those designated sector type “A” and those designatedsector type “B”, with sector categories alternating within a cell sothat no two adjacent cells fall in the same category and with cellsarranged so that no two adjacent sectors from adjoining cells fall inthe same category. Each sector is falls within a different category thanall other adjacent sectors with which the respective sector shares acommon linear boundary.

FIGS. 3C through 3E are high level timing diagrams illustratingbandwidth allocation among sectors according to one embodiment of thepresent disclosure, and are intended to be read in conjunction with FIG.2A. The present disclosure incorporates FDD operation, with dedicateddownlink and uplink channels, within a TDD system by introducing afrequency change at the normal. TDD guard point. Transmission time onthe dedicated downlink frequency F1 and the dedicated uplink frequencyF2 are divided between adjacent sectors within Categories A and B. Thus,the TDD FDD system 320 of the present disclosure allocates both adownlink period 321, 322 on the downlink frequency F1 and an uplinkperiod 323, 324 on the uplink frequency F2 to each of the sectors withincategories A and B.

The allocated periods 312/322 and 323/324 are offset in both time andfrequency, then overlaid so that the sector A downlink period 321 doesnot coincide in time or frequency with the sector A uplink period 324and the sector B downlink period 322 does not coincide in time orfrequency with sector B uplink period 323. Instead, downlinktransmission 321 in each sector within category A occurs at the sametime as uplink transmission 323 within each sector within category B,while downlink transmission 322 in each sector within category B occursconcurrently with uplink transmission 324 for each sector withincategory A.

In this manner, the dedicated downlink frequency F1 and the dedicateduplink frequency F2 are time-shared by adjacent sectors, but remaindedicated to downlink or uplink transmission and may utilize FDD-onlybandwidth within the MMDS spectrum. Duplex spacing 313 between downlinkand uplink frequencies F1 and F2 (typically 50-70 MHz) is alsomaintained.

FIG. 4 depicts in greater detail a frame structure employed within theexemplary bandwidth allocation scheme according to one embodiment of thepresent disclosure, and is intended to be read in conjunction with FIGS.2 and 3C through 3E. The frame 400 depicted corresponds to each of thesectors within category A described above and depicted in FIGS. 2A and3C through 3E, although each sector within category would utilize asimilar frame, as described in further detail below.

Frame 400 includes a frame header 410, a downlink sub-frame 420, and anuplink sub-frame 430, with the downlink and uplink sub-frames logicallydivided into a number of physical slots 440. The frame header 410includes a preamble 411 containing a start-of-frame field, which allowssubscribers using fixed diversity to test reception conditions of thetwo diversity antennas, and a physical layer (the air interface islayered as a physical layer and a media access layer) media dependentconvergence field, utilized to assist in synchronization andtime/frequency recovery at the receiver. The preamble 411 is followed bymedia access management information 412, which includes a downlink MAPidentifying the physical slot where the downlink ends and the uplinkbegins, an uplink MAP indicating uplink subscriber access grants and theassociated physical slot start of the grant, and other managementmessages such as acknowledge (ACK) response, etc.

During the downlink sub-frame 420, the base transmitter and thesubscriber receiver are both set to the downlink frequency F1. Thedownlink sub-frame 420 terminates with a frequency change physical slot421, during which multi-stage digital filters within both the base andthe subscriber unit are altered to switch to the uplink frequency F2,followed by a transmitter transition guard time 422, during which notransmission occurs to allow for propagation delays for all subscriberunits. The transmitter transition guard time 422, depicted as occupyingthree physical slots in FIG. 4, is fully programmable both in positionand duration, set by management physical layer attribute messages.

During the downlink sub-frame 430, the base receiver and the subscribertransmitter(s) are both set to the uplink frequency F2. The firstphysical slots within the uplink sub-frame 430 are subscriberregistration or acquisition uplink ranging slots, utilized for bothinitial uplink synchronization of subscribers performing entry into thenetwork and periodic update of synchronization of active subscribers,followed by contention slots, providing a demand access requestmechanism to establish subscriber service for a single traffic serviceflow. When collisions occur within the contention slots, the subscriberemploys a random back-off in integer frame periods and retries based ona time out for request of service. Contention slots use the lowestpossible modulation, forward error correction (FEC), and orthogonalexpansion supported by the base. The number and position of registrationand contention slots within the uplink sub-frame 430 is set by theuplink MAP message within the media access management informationportion 412 of the frame header 410.

The contention slots within the uplink sub-frame 430 are followed byindividual subscriber transmissions which have been scheduled andallocated by the base in the uplink MAP, with each subscribertransmission burst performed at the maximum modulation, FEC andorthogonal expansion supported by the subscriber unit. The uplinksub-frame 430 terminates with a frequency change physical slot 431,during which both the base and the subscriber unit switch to thedownlink frequency F1, followed by a receiver transition guard time 432,which is also programmable.

Frames for sectors falling within category B will have a similarstructure, but will be offset so that the downlink sub-frame of eachcategory B sector corresponds in time with the uplink sub-frame of eachcategory A sector, and the uplink sub-frame of each category B sectorcorresponds in time with the downlink sub-frame of each category Asector. The boundary between downlink and uplink sub-frames is adaptiveutilizing block equalization and burst timing coordination. Accordingly,uplink and downlink allocations to sectors in categories A and B may bedivided equally as shown in FIG. 3C. or may be split to allow greatertime within a particular frame to the downlink for sectors in categoryA, as shown in FIG. 3D, or to the downlink for sectors in category B, asshown in FIG. 3E. Spectral efficiency is therefore improved by adaptingto the instantaneous traffic requirements among various sectors.

While the exemplary embodiment is described above with six sector cellsand only two sector categories for the purposes of simplicity andclarity in describing the disclosure, the present disclosure may beextended to any number of sector categories equal to a power of 2 (e.g.,2, 4, 8, . . . , etc.), and preferably employs four sector categories.Where more than two sector categories are employed, downlink and uplinkfrequencies may be reused in pairs or in staggered offsets (e.g., eachsector A shares a downlink frequency F1 with one adjacent sector B butshares an uplink frequency F2 with a different adjacent sector C, etc.).FIG. 2B depicts a cell and sector layout for a wireless access coveragearea according to an alternative embodiment of the present disclosure.Coverage area 250 is logically divided into cells 260, 270, 280 and 290each logically divided into four sectors 261-264, 271-274, 281-284 and291-294, respectively. Each cell 260, 270, 280 and 290 includes atransceiver base station 110 as depicted in 28 FIG. 1 at a centrallocation 265, 275, 285, and 295, as well as subscriber premises 121-123within the coverage area of the respective cell.

Sectors 261-264, 271-274, 281-284 and 291-294 in the alternativeembodiment are logically divided into four categories, designated sectortype “A”, “B”, “C” and “D”, with sector categories arranged within acell and between cells so that no two adjacent cells fall in the samecategory and no cell adjoins two or more cells in the same category.Each sector falls within a different category than all other adjacentsectors with which the respective sector shares a common linearboundary.

FIG. 5 is functional diagram of filtering employed for wirelesscommunication within each cell in accordance with one embodiment of thepresent disclosure, and is intended to be read in conjunction with FIGS.1, 2A-2B, 3C-3E, and 4. The filtering system 500 depicted is implementedwithin each transceiver base station 110 and each subscriber accessdevice on subscriber premises 121-123. The parameters for filteringsystem 500 implemented within each subscriber premises 121-123 will bedescribed, although those skilled in the art will recognize that thefiltering systems within each transceiver base station 110 will simplyhave the transmission and reception frequencies downlink or uplinkfrequencies F1 and F2) reversed or otherwise changed.

Wireless signals at the appropriate downlink and uplink frequencies F1and F2 for the subject cell and sector are transmitted and received viaantenna 501 and separated by a diplexer 502. Signals received from orpassed to diplexer 502 are filtered utilizing filters 503 and 504 tunedto downlink and uplink frequencies F1 and F2, respectively. The signalreceived from filter 503 is mixed with a signal from a local oscillator505 tuned to the downlink frequency F1 f while the signal transmitted tofilter 504 is mixed with a signal from a local oscillator 506 tuned tothe uplink frequency F2. If direct conversion is utilized, the output ofmixer 507 may be connected directly to analog-to-digital (A/D) converter508, and the input to mixer 509 may be connected directly todigital-to-analog (D/A) convert 510.

If super heterodyne conversion is employed, as is preferable, filteringsystem 500 includes a second (optional) conversion stage 511. Withinconversion stage 5111 the output of mixer 507 passes to a filter 512tuned to an image frequency based on the downlink frequency F1, with thefiltered output being mixed with a signal from a local oscillator 513also tuned to the image frequency based on downlink frequency F1 beforebeing passed to A/D converter 508. Similarly, signals from D/A converter510 are mixed with a signal from a local oscillator 514 tuned to animage frequency based on the uplink frequency F2 and is passed through afilter 515 also tuned to the image frequency based on the uplinkfrequency F2 before being passed to mixer 509.

A/D and D/A converters 508 and 510 are coupled to a digitalmodulator/demodulator 516 which decodes and generates the digitalsignals from the wireless communications downlinks and uplinks.Additional digital filtering 517 may optionally be employed between A/Dconverter 508 and modulator/demodulator 516. The filters 503, 504, 512and 515, mixers 507, 509, 518 and 519, A/D/ and D/A converters 508 and510, digital filter 517, and digital modulator/demodulator 516 may beimplemented in either hardware or software, collectively, individually,or in any combination of the individual elements.

Filtering system 500 should have two essential characteristics forsuccessful implementation of a TDD FDD system in accordance with oneembodiment of the present disclosure. First, the frequency switchingtime between the uplink and downlink frequencies for the filteringsystem 500 within all transceivers (within each transceiver base station110 and each subscriber premises 121-123) must be sufficiently fast tocomplete during the frequency change physical slots 421 and 431.Frequency change physical slots 421 and 431, together with guard times422 and 432, insure that transmission of an uplink/downlink sub-frame iscompleted successfully before transmission of the next sub-frame isstarted. Frequency switching should preferably take no longer than ¼ to1/10 the duration of physical slots 421 and 431. Physical slots 421 and431 and/or guard times 422 and 432 may alternatively be extended induration to accommodate longer frequency switching times within atransceiver between the downlink and uplink frequencies.

Second, filtering system 500 must filter transmitted and receivedsignals in depth to ensure, in conjunction with the duplex spacingemployed between the downlink and uplink frequencies F1 and F2, thatspurious out-of-band transmission products do not interfere with thereceiver. FIG. 6 illustrates a spectral response for filtering employedfor wireless communication within each cell and sector in accordancewith one embodiment of the present disclosure. A signal strength 600 atwhich unacceptable interference prevents successful communication may beidentified or defined for a particular system. Filtering system 500should pass signals within the band 601 allocated to downlink frequencyF1 and within the band 602 allocated to uplink frequency F2. By virtueof duplex spacing 313 between the downlink and uplink frequencies F1 andF2, together with the in-depth filtering performed by filtering system500, out-of-band signals are sufficiently rejected to prevent the signalstrength from approaching interference level 600.

FIG. 7 is functional diagram of filtering employed for wirelesscommunication within each cell and sector in accordance with anotherembodiment of the present disclosure. Filtering system 700 receiveswireless signals at the appropriate downlink and uplink frequencies F1and F2 for the subject cell and sector via antenna 501. Signals receivedfrom or passed to antenna 501 are filtered utilizing filter 701, whichcovers the full FDD band employed for the subject sector. A switch 702selective connects the filter 701 to a power amplifier (PA) 703 fortransmission or to a low noise amplifier (LNA) 704 for reception.

In the embodiment depicted in FIG. 7, the conversion stages coupled topower amplifier 703 and low noise amplifier 704 are bidirectional, andas a result of the TDD aspect of the signal pattern employed may bereused for both transmitting and receiving signals. Local oscillator 705coupled to mixer 706 should be capable of switching frequencies,converting signals at either the downlink frequency F1 or the uplinkfrequency F2 to an image frequency. Optional second stage 707 forsuperheterodyne conversion includes a filter 708 and local oscillator709 both tuned to the image frequency and a mixer 710. A/D converter 508and D/A converter 510 are both connected to mixer 710.

The FDD. TDD strategy of the present disclosure permits filtering andconversion to be performed along a single, bidirectional signal pathwhich is reused for both the downlink and the uplink, eliminating theneed for separate paths and reducing the system costs. The spectralperformance illustrated in FIG. 6 should be implemented by filteringsystem 700, with the frequency switching time for local oscillator 705within the first conversion stage being critical to meeting the timingrequirements imposed by the FDD TDD system of the present disclosure.

It is important to note that while the present disclosure has beendescribed in the context of a fully functional data processing systemand/or network, those skilled in the art will appreciate that themechanism of the present disclosure is capable of being distributed inthe form of a computer usable medium of instructions in a variety offorms, and that the present disclosure applies equally regardless of theparticular type of signal bearing medium used to actually carry out thedistribution. Examples of computer usable mediums include: nonvolatile,hard-coded type mediums such as read only memories (ROMs) or erasable,electrically programmable read only memories (EEPROMs), recordable typemediums such as floppy disks, hard disk drives and CD-ROMs, andtransmission type mediums such as digital and analog communicationlinks.

Although the present disclosure has been described in detail, thoseskilled in the art should understand that they can make various changes,substitutions and alterations herein without departing from the spiritand scope of the disclosure in its broadest form.

1. A method of TDD operation in a first subscriber unit having a cablemodem, the method comprising: receiving in the first subscriber unit afirst signal from a first base station on a downlink frequency during afirst time period; and transmitting from the first subscriber unit asecond signal to the first base station on an uplink frequency during asecond time period following the first time period, wherein the downlinkfrequency and the uplink frequency are separated by a predefined duplexspacing.
 2. A cable modem configured to use TDD in a first subscriberstation, the cable modem comprising: a receiver circuit configured toreceive a first signal on a first frequency designated for downlinktransmission during a first time period; and a transmitter circuitconfigured to transmit a second signal on a second frequency differentfrom the first frequency and designated for uplink transmission during asecond time period following the first time period, wherein the firstfrequency is employed for downlink transmission to a second subscriberstation during the second time period and the second frequency isemployed for uplink transmission from the second subscriber stationduring the first time period.
 3. A method of time sharing frequenciesreserved for FDD operation, the method comprising: receiving in a cablemodem a first signal from a first base station on a downlink frequencyduring a first time period; and transmitting from the cable modem asecond signal to the first base station on an uplink frequency during asecond time period following the first time period, wherein the downlinkfrequency and the uplink frequency are separated by a predefined duplexspacing.
 4. A cable modem configured to use TDD, the modem comprising: atransmitter circuit configured to transmit a first signal on a firstfrequency to a first subscriber station during a first time period, andtransmit a second signal all a second frequency, different form thefirst frequency, to a second subscriber station during a second timeperiod following the first time period; and a receiver circuitconfigured to receive a third signal on the second frequency from thesecond subscriber station during the first time period, and receive afourth signal on the first frequency from the first subscriber duringthe second time period.